MIRROR NEURONS AND AUTISM

A mirror neuron is a neuron which fires both when
an animal performs an action and when the animal observes the same
action performed by another (especially conspecific) animal. Thus,
the neuron "mirrors" the behavior of another animal, as
though the observer were itself performing the action. These neurons
have been directly observed in primates, and are believed to exist
in humans and in some birds. In humans, brain activity consistent
with mirror neurons has been found in the premotor cortex and the
inferior parietal cortex. Some scientists consider mirror neurons
one of the most important findings of neuroscience in the last decade.
Among them is V.S. Ramachandran1, who believes
they might be very important in imitation and language acquisition.
However, despite the popularity of this field, to date no plausible
neural or computational models have been put forward to describe
how mirror neuron activity supports cognitive functions such as
imitation.

Introduction

In the monkey, mirror neurons are found in the
ventral premotor cortex (probably the equivalent of the inferior
frontal gyrus in humans) and in the anterior inferior parietal lobule.
These neurons are active when the monkeys perform certain tasks,
but they also fire when the monkeys watch or hear someone else perform
the same specific task. Researchers using fMRI, TMS, and EEG have
found evidence of a similar system (matching observations with actions),
in the human brain.

The function of the mirror system is a subject
of much speculation. These neurons may be important for understanding
the actions of other people, and for learning new skills by imitation.
Some researchers also speculate that mirror systems may simulate
observed actions, and thus contribute to our theory of mind skills,
2 3 while others relate mirror neurons to
language abilities. 4 It has also been proposed
that problems with the mirror system may underlie cognitive disorders,
in particular autism. 5 6

Research into all of these possibilities is ongoing.

Discovery

In the 1980s and 1990s, Giacomo Rizzolatti was
working with Luciano Fadiga, Leonardo Fogassi and Vittorio Gallese
at the university in Parma, Italy. These scientists had placed electrodes
in the inferior frontal cortex of the macaque monkey to study neurons
specialised for the control of hand actions, for example, grabbing
objects, picking items up etc. During each experiment, they recorded
from a single neuron in the monkey's brain while the monkey was
allowed to reach for pieces of food, so the researchers could measure
the neuron's response to certain movements. 7

This work has since been published 8
and confirmed 9 with mirror neurons found
in both inferior frontal and inferior parietal regions of the brain.
Recently, evidence from fMRI, TMS and EEG and behavioral strongly
suggest the presence of similar systems in humans, where brain regions
which respond during both action and the observation of action have
been identified. Not surprisingly, these brain regions closely match
those found in the macaque monkey 10.

More recently Keysers and colleagues have shown
that both in humans and monkeys, the mirror system also responds
to the sound of actions 11 12.

Mirror neurons in monkeys

The only animal where mirror neurons have been
studied individually is the macaque monkey. In these monkeys, mirror
neurons are found in the inferior frontal gyrus (region F5) and
the inferior parietal lobule. 13

Mirror neurons are believed to mediate the understanding
of other animal's behavior. For example, a mirror neuron which fires
when the monkey rips a piece of paper would also fire when the monkey
sees a person rip paper, or hears paper ripping (without visual
information). These properties have led researchers to believe that
mirror neurons encode abstract concepts of actions like 'ripping
paper', whether the action is performed by the monkey or another
animal. 14

The function of mirror neurons in macaques is
not known. Adult macaques do not seem to learn by imitation. Recent
experiments suggest that infant macaqes can imitate a human's face
movements, only as neonates and during a limited temporal window
15. However, it is not known if mirror neurons
underlie this behaviour.

In adult monkeys, mirror neurons may enable the
monkey to understand what another monkey is doing, or to recognise
the other monkey's action. 16

The mirror neuron system in humans

It is not normally possible to study single neurons
in the human brain, so scientists can not be certain that humans
have mirror neurons. However, the results of brain imaging experiments
have shown that the human inferior frontal cortex and superior parietal
lobule is active when the person performs an action and also when
the person sees another individual performing an action. Therefore,
these brain regions are likely to contain mirror neurons and have
been defined as the human mirror neuron system. 17
Human infant data suggest that the mirror neuron system develops
before 12 months of age, and that this system helps human infants
understand other peoples' actions. 18

Many different functions for the mirror neuron
system have been suggested. These include:

Understanding Intentions

Many studies link mirror neurons to understanding
goals and intentions. Fogassi et al. (2005) 19 recorded
the activity of 41 mirror neurons in the inferior parietal lobe
(IPL) of two rhesus macaques. The IPL has long been recognized as
an association cortex that integrates sensory information. The monkeys
watched an experimenter either grasp an apple and bring it to his
mouth or grasp an object and place it in a cup. In total, 15 mirror
neurons fired vigorously when the monkey observed the "grasp-to-eat"
motion, but registered no activity while exposed to the "grasp-to-place"
condition. For four other mirror neurons, the reverse held true:
they activated in response to the experimenter eventually placing
the apple in the cup but not to eating it. Only the type of action,
and not the kinematic force with which models manipulated objects,
determined neuron activity. Significantly, neurons fired before
the monkey observed the human model starting the second motor act
(bringing the object to the mouth or placing it in a cup). Therefore,
IPL neurons "code the same act (grasping) in a different way
according to the final goal of the action in which the act is embedded"
(664). They may furnish a neural basis for predicting another individual’s
subsequent actions and inferring intention.

Empathy

Mirror neurons have been linked to empathy, because
certain brain regions (in particular the anterior insula and inferior
frontal cortex) are active when a person experience an emotion (disgust,
happiness, pain etc) and when they see another person experience
an emotion. 21 22 23 However, these brain
regions are not quite the same as the ones which mirror hand actions,
and mirror neurons for emotional states or empathy have not yet
been described in monkeys. More recently, Keysers and colleagues
have shown that people that are more empathic according to self-report
questionnaires have stronger activations both in the mirror system
for hand actions 23 and the mirror system
for emotions 24 providing more direct support
to the idea that the mirror system is linked to empathy.

Language

In humans, mirror neurons have been found in the
inferior frontal cortex, close to Broca's area, a language region.
This has lead to suggestions that human language evolved from a
gesture performance/understanding system implemented in mirror neurons.
Mirror neurons certainly have the potential to provide a mechanism
for action understanding, imitation learning, and the simulation
of other people's behaviour. 25 However,
like many theories of language evolution, there is little direct
evidence either way.

Autism

Some researchers claim there is a link between
mirror neuron deficiency and Autism.
In typical children, EEG recordings from motor areas are suppressed
when the child watches another person move, and this is believed
to be an index of mirror neuron activity. However, this suppression
is not seen in children with Autism 26. Also,
children with Autism have less activity in mirror neuron regions
of the brain when imitating 27. Based on
these results, some researchers claim that Autism is caused by a
lack of mirror neurons, leading to disabilities in social skills,
imitation, empathy and theory of mind. This is just one of many
theories of Autism and it has not yet been proven.

Theory of Mind

In Philosophy of mind, mirror neurons have become
the primary rallying call of simulation theorists concerning our
'theory of mind.' 'Theory
of mind' refers to our ability to infer another person's mental
state (i.e., beliefs and desires) from their experiences or their
behavior. For example, if you see a person reaching into a jar labelled
'cookies,' you might assume that he wants a cookie (even if you
know the jar is empty) and that he believes there are cookies in
the jar.

There are several competing models which attempt
to account for our theory of mind; the most notable in relation
to mirror neurons is simulation theory. According to simulation
theory, theory of mind is available because we subconsciously put
ourselves in the shoes of the person we're observing and, accounting
for relevant differences, imagine what we would desire and believe
in that scenario. 28 29 Mirror neurons have
been interpreted as the mechanism by which we simulate others in
order to better understand them, and therefore their discovery has
been taken by some as a validation of simulation theory (which appeared
a decade before the discovery of mirror neurons). 30

Gender differences

Stronger EEG responses related to the mirror neuron
system have been recorded in women compared to men 31.
This finding is consistent with the idea that women tend to be more
empathetic, that the mirror neuron system is related to empathy,
and that weak responses in the mirror neuron system could be linked
to a masculine mind and to autism 32. However,
these ideas have not been tested in full.